The duel!

Situation
Situation puzzles (sometimes called lateral thinking puzzles) are ones where you need to ask lots of yes or no questions to figure out what happened in the situation. These are good puzzles for groups where one person knows the puzzle and answers the questions.

Two men, John and Frank, fought a duel with guns. Each shot their opponent in the head, but neither John nor Frank died. How is this possible?

They could've been having a duel in a bathroom on a ship (aka 'head'), and each of them happened to shoot the other non-lethally, in the leg or something. I wouldn't be surprised if their aim was off in a cramped space like that.